274 i The Process of Evolution 



EVOLUTIONARY TRENDS 



In retrospect one can observe evolutionary trends of virtually any 

 desired degree of generality, ranging from a trend toward increased 

 melanism in a British moth to a trend toward increasing complexity 

 in the biosphere. Unfortunately such trends observed a posteriori 

 have all too often been interpreted as if the evolutionary process was 

 a means to a predetermined end. The fallacy of this view may be ex- 

 posed with a very crude analogy. A driver leaving New York City 

 might, at each intersection he encountered, take the fork that seemed 

 to promise the easiest going ( based upon what he could observe at 

 the intersection about the amount of traffic and condition of the 

 road). At the end of a week of this whimsical travel he might be 

 found on the Indiana toll road. A teleologically oriented observer 

 would doubtless claim that someone had planned a trip to Chicago 

 for him. Readers interested in the sorts of trends that have been ob- 

 served in the fossil record are referred to the works of Rensch and 

 Simpson. These writers are highly successful in interpreting these 

 trends in the light of the basic mechanisms of mutation, recombina- 

 tion, selection, and drift. 



Increase in Size 



Two very general trends are considered briefly here and in the fol- 

 lowing section. In many phyletic lines of animals, there is a tend- 

 ency toward increase in size. (This has been called Cope's rule.) 

 The validity of this generality, in the broadest terms, can be ques- 

 tioned only if one is willing to assume that the earliest living organ- 

 isms were above or near the middle of the present size spectrum. 

 From the discussion of the origin of life ( Chap. 1 ) it will be seen 

 that this was hardly likely. Possible selective advantages of being 

 large can be found in virtually every physiological function of or- 

 ganisms: maximum exposure of leaves to photosynthesis (trees), heat 

 conservation (whales), room for complex brains (pigs), or resistance 

 to desiccation by a low surface-volume relation ( barrel cacti ) . 



The fossil record shows many trends toward larger size. A well- 

 documented instance is that of the phylogeny of horses, vdth dog- 

 sized Eocene horses giving rise to, among others, the contemporary 

 work horses. The mammal record is replete with similar stories, al- 

 though exceptions are known (e.g., modern marsupials and sloths 

 are smaller than their Pleistocene counterparts ) . It is necessary, but 

 sometimes difficult, to differentiate between reversal of a size-change 



